Aromatic carboxylic acids are commonly manufactured by catalytically oxidizing aromatic alkyls in the liquid phase under elevated temperature and pressure conditions. U.S. Pat. Nos. 2,245,528; 2,833,816; 3,092,658 and 3,170,768, the disclosures of which are incorporated herein by reference, are illustrative of the manufacturing process. Typically, the medium within the oxidation reactor includes the aromatic alkyl, the oxidation catalyst, an oxygen-containing gas, and a solvent, typically a lower aliphatic monocarboxylic acid.
A liquid product stream from the reactor contains in addition to the aromatic carboxylic acid, the oxidation catalyst, solvent, oxidation reaction by-products, and other process impurities. The oxidation catalyst typically consists of one or more of cobalt, manganese, and hydrogen bromide.
After cooling the product stream to crystalize the product aromatic acid, the reactor product stream is passed through a separation process to remove a major portion of the product aromatic carboxylic acid. The commonly employed means of separation is by centrifugation. The mother liquor is then returned to the oxidation reactor. A small residue purge stream, which contains the oxidation catalyst, minor residual aromatic carboxylic acids, reaction by-products, solvent and process impurities, is sent to a separation process for recovery of the solvent. This separation process produces a concentrated sludge containing the oxidation catalyst, reaction by-products and process impurities.
Considerable efforts have been directed toward removing the impurities from the previously mentioned catalyst-containing purge stream using conventional separation techniques. However, all such known methods to effectively remove the impurities from the process stream are technically impractical or economically undesirable. Such methods include chemical precipitation or incineration followed by hydrometallurgical recovery of the catalyst. Known methods using ion exchange resins do not incorporate an upstream filtering scheme for removing fine product particulate matter that degrades the function of the ion exchange resins.
U.S. Pat. No. 2,964,559 discloses a process involving liquid phase oxidation for recovering heavy metal oxidation catalysts by extracting the catalyst from distillation bottoms with a solvent.
U.S. Pat. No. 3,341,470 discloses a process for the recovery of cobalt and manganese catalysts from an oxidation reaction mixture by incinerating the stream to convert the various metals to their oxides and effecting selective chemical precipitation of the contaminants with specific reagents.
U.S. Pat. No. 3,873,468 discloses aqueous extraction from still bottoms followed by carbonate precipitation.
U.S. Pat. No. 3,959,449 also discloses an aqueous extraction from still bottoms but followed by a strongly acidic cation-exchange resin, distilling the solution to recover bromine as hydrobromic acid, and recovering the heavy metal catalysts as bromides.
U.S. Pat. No. 4,162,991 discloses recovery of cobalt and bromide catalysts by absorbing the catalysts on a strongly basic anion exchange resin and desorbing cobalt and bromide ions with lower aliphatic monocarboxylic acid.
U.S. Pat. Nos. 4,546,202; 4,266,084; 4,258,227; and 4,393,264 disclose the recovery of catalysts from pyrolysis ash.
U.S. Pat. No. 4,855,491 discloses a method employing a nanofilter to pass sodium benzoate and reject Cobalt catalysts.
U.S. Pat. No. 4,238,294 discloses a method for recovering heavy metal ions and halogen values using anion exchange resins. The disclosed process requires that the process stream's water concentration be 20% or less by weight. The disclosed process does not incorporate any filtration process to remove particulate matter prior to ion exchange using the anion exchange resins.
Due to the enormous quantities of various aromatic carboxylic acid products now manufactured for various uses, and the high cost of catalyst materials for such processes, it is desirable to provide a cost-effective process for recovering and reusing the oxidation catalyst. The present invention provides a significant improvement over current processes by efficiently and economically recovering the expensive catalyst materials for reuse. There is also a need for a separation process which effectively separates and recovers the insoluble aromatic acid product and economically removes oxidation catalyst from the residue purge stream and recovers and recycles the catalyst to the oxidation process and at the same time allowing the undesirable impurities to remain in the residue purge stream. An additional economic benefit of this invention is the further purification of the residual aromatic acids in the purge stream which enables the manufacturer of aromatic acids to obtain some commercial value from the residual aromatic acids that are currently lost via waste disposal processes.